This application claims priority from German patent application DE 199 42 055.6-45 filed Sep. 3, 1999.
The invention pertains to a method for printing on a thermoplastic material, where a coloring agent is applied to the surface of the material and is subsequently caused to harden.
Various printing methods are known from the prior art, for example, silk screen, tampon printing or the flexoprint method. With these methods, solvent-containing dye is applied onto the thermoplastic material. The solvent is evaporated, so that the dye can harden. Some time after manufacture of the thermoplastic material, chemical reactions occur between the material surface and the formerly very difficult to scrape off dye, which adversely affect adhesion.
It is an object of the invention to create a method of the kind described above, wherein permanent coloration can be achieved on the material to be printed.
The invention involves a method whereby as coloring agent, a toner featuring thermoplastic toner particles, is applied by electrographic or electrostatic means to the surface of the material, and that the toner and/or at least a portion of the surface of the material is brought into a reactive state in that the toner makes a permanent bond with the surface.
The thermoplastic toner particles enter into a permanent bond with the material, which later, in the hardened state, cannot be readily relaxed again. Thus, permanent bonding of the coloring agent can be achieved. Because the coloring agent is applied in the form of a toner by electrographic or electrostatic means onto the material, a flexible printing process is possible, since even complicated printed patterns can be produced in very small lot sizes.
According to one preferred configuration variant of the invention, this method provides in particular for the surface of the material to be brought into a fluid or dough-like state by means of thermal energy.
One possible variant of the invention is characterized in that the thermoplastic material is processed in a molding machine under the influence of temperature, that the processed material is additionally heated at least in regions of its surface to produce the reactive state, or is kept in this reactive state under the influence of temperature, and that the toner is subsequently applied to the surface to be printed. Thus, in this case the printing process will directly follow the molding process.
According to another method of this invention, it is also provided that the thermoplastic material be processed in a molding machine under the influence of temperature, that subsequent to the molding process, the processed material be caused to harden (state Z1), and that the hardened material subsequently be sent to a warming device and brought into the reactive state at least in regions of its surface.
With regard to the two aforementioned methods, the toner can be brought to the reactive state by means of a warming device, or the toner can be brought into the reactive state by the thermal energy present in the material.
In addition, a process control is possible which is disconnected from the actual molding process. In this case, the invention provides that the thermoplastic material be processed in a molding machine under the influence of temperature, that subsequent to the molding process, the processed material be caused to harden (state Z1), that the toner be brought into the reactive state in a warming device before application to the surface of the material to be coated, and that the surface of the heated toner be partially brought into the reactive state.
In order to ensure dependable hardening of the coloring agent, the invention also provides that subsequent to the coloration process, after the coloring agent has bonded with the surface of the material, said material is brought into the hardened state together with the coloring agent in a cooling section.
As a result of the printing process, in order to cause little or no effect on the surface quality of the material, yet another method according to this invention provides that the coloring agent be sunken into the surface of the material to form a smooth surface structure. In this way, a homogeneous, smooth surface can be produced.
Preferably, the thermoplastic toner particles are of the same thermoplastic material as the material to be coated.